Up until now, there have been proposed a wide variety of semiconductor optical elements of this type, one typical example of which needs to include a waveguide having one end surface with low reflectivity and one end surface with high reflectivity. Each of the semiconductor optical elements is to be applied to an optical device such as an external cavity laser diode (EC-LD) and a superluminescent diode (SLD).
To reduce the reflectivity of the end surface of the waveguide, the following waveguides have been proposed: a waveguide having a window structure at the end surface thereof; and a waveguide including an end portion having an optical axis, which is not perpendicular to the end surface thereof.
The waveguide provided with the window structure, however, may have a crystal defect in the vicinity of the window structure. The occurrence of the crystal defect may reduce reliability of a semiconductor optical element having the waveguide.
To avoid the above problem, there has been proposed a semiconductor optical element including a waveguide having an optical axis, which is not perpendicular to an end surface of the semiconductor optical element (refer to, for example, Patent Documents 1 and 2).
FIG. 13(a) is a top view of a semiconductor optical amplifier (SOA) 110, which is one of semiconductor elements disclosed in Patent Document 1. The semiconductor optical amplifier 110 includes a linear waveguide 110G having an optical axis, which is not perpendicular to a cleavage surface 110G1 and a cleavage surface 110G2 of the semiconductor optical amplifier 110. Patent Document 1 also discloses that reflectivity of a traveling-wave laser amplifier is reduced by flaring a waveguiding portion of the amplifier in the immediate vicinities of the end surfaces of the waveguide 110G.
FIG. 13(b) is a top view of an optical device 130 having an EC-LD 120, which is one of semiconductor elements disclosed in Patent Document 2. The EC-LD 120 has cleavage surfaces 120G1 and 120G2 and includes a waveguide 120G having an end surface low reflectivity and an end surface with high reflectivity. The low-reflectivity end surface of the waveguide 120G is inclined at a predetermined angle with respect to the cleavage surface 120G1, while the high-reflectivity end surface of the waveguide 120G is perpendicular to the cleavage surface 120G2.
In the optical device 130, light is generated in the waveguide 120G, reflected on the high-reflectivity end surface of the waveguide 120G, and returned to the inside of the waveguide 120G. The light then reaches a diffraction grating 122 through a collimator lens 121.
Among the light diffracted by the diffraction grating 122, only light having a wavelength satisfying a predetermined condition is reflected on a recursive mirror 123, diffracted by the diffraction grating 122 again, and returned to the EC-LD 120. The optical device 130 is designed to select a frequency of laser oscillation. The light is output from the optical device 130 to the side opposite to the EC-LD 120 through diffraction on the face of the diffraction grating 122.
Patent Document 1: U.S. Pat. No. 4,965,525 (pages 2 to 5, FIG. 1)
Patent Document 2: U.S. Pat. No. 6,091,755 (page 5, FIG. 2)